REDUCTION OF SiCl4 IN THE PRESENCE OF BCl3

ABSTRACT

The present invention relates, in general, to the purification of boron trichloride (BCl 3 ). More particularly, the invention relates to a process for minimizing silicon tetrachloride (SiCl 4 ) formation in BCl 3  production and/or the removal of SiCl 4  in BCl 3  product stream by preventing/minimizing the silicon source in the reaction chambers. In addition, a hydride material may be used to convert any SiCl 4  present to SiH 4  which is easier to remove. Lastly freeze separation would replace fractional distillation to remove SiCl 4  from BCl 3  that has been partially purified to remove light boilers.

CROSS REFERENCE TO OTHER APPLICATIONS

This application claims benefit of priority to U.S. ProvisionalApplication No. 61/954,599, filed Mar. 18, 2014, the disclosure of whichis fully incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved column reactor device andprocesses for the purification of boron trichloride (BCl₃). Moreparticularly, the present invention relates to a device that minimizessilicon tetrachloride (SiCl₄) formation during BCl₃ production, anddiscloses processes for the removal of SiCl₄ from the BCl₃ productstream which may have been formed during the synthesis of BCl₃.

2. Description of the State of the Art

Boron trichloride (BCl₃) is a highly reactive compound packaged as aliquid under its own vapor pressure that has numerous diverseapplications. It is used predominantly as a source of boron in a varietyof manufacturing processes. For example, in the manufacturing ofstructural materials, BCl₃ is the precursor for chemical vapordeposition (“CVD”) of boron filaments used to reinforce high performancecomposite materials. BCl₃ is also used as a CVD precursor in the borondoping of optical fibers, scratch resistant coatings, andsemiconductors. Some of the non-CVD applications of BCl₃ are reactiveion etching of semiconductor integrated circuits and refining of metalalloys. In metallurgical applications, it is used to remove oxides,carbides, and nitrides from molten metals. In particular, BCl₃ is usedto refine aluminum and its alloys to improve tensile strength.

There are known a number of processes for the production of BCl₃ forexample, by chlorination of a borate ester, e.g., trimethyl borate, in asealed tube. See, for example, U.S. Pat. No. 2,943,916. However, themost common technical process for the preparation of BCl₃ is thereaction of a boron compound, such as boron carbide (B₄C) with chlorine.In this process BCl₃ can be prepared by passing chlorine over mixturesof boron carbide and optionally carbon, packed within a quartz column,which is heated to elevated temperatures of at least 800° C. to 1,200°C. Once the reaction is established, the reaction zone propagates slowlydown the column generating BCl₃ at the reaction zone. The chlorinationreaction results in the formation of BCl₃ having impurities such asunreacted chlorine (Cl₂), hydrogen chloride (HCl), and phosgene (COCl₂)which are generally removed from the raw BCl₃ stream throughdistillation and/or other purification methods. However, trace amountsof silicon tetrachloride (SiCl₄) are also produced and are much moredifficult to remove from the product stream by the above described meansdue to its low volatility. The crude product, i.e., BCl₃ containing theSiCl₄ byproduct, is useful for some purposes; but, for many uses, SiCl₄is an undesirable impurity, e.g., when BCl₃ is used as a precursor forhigh purity boron nitride. Therefore, its minimization during synthesisin the packed column reactor and its removal from the resulting BCl₃product stream is highly desirable.

Boron trioxide (B₂O₃) typically exists in boron carbide as an impuritywith content varying from 1% (wt) to 5% (wt). Boron trioxide has amelting point temperature of about 450° C. or about 510° C. depending onits crystal structure. Hence, under the reaction condition as mentionedabove, the impurity B₂O₃ in B₄C melts and forms a liquid in the B₄Cchlorination process. The liquidized B₂O₃ in the process streameventually forms deposits as the process temperature is below itsmelting point. The deposits may block the process stream flow as theyare continuously accumulated after multiple reaction cycles. Typically,an activated carbon (such as charcoal) bed is loaded at the bottom ofthe reactor to adsorb liquidized B₂O₃. In the major section of thereactor, once the reaction is triggered, through induction heating, aporous carbon frame (graphite) is formed after boron is chlorinated anddepleted from B₄C. The presence of carbon (the carbon in the activatedcarbon bed and the carbon formed during the chlorination process) has adetrimental impact on BCl₃ purity, i.e., carbon can enhance thechlorination of quartz (SiO₂) at the B₄C chlorination temperature ofleast 800° C. to 1,200° C. resulting in the formation of SiCl₄, a highlyundesirable impurity in BCl₃, according to the reaction below:

SiO₂+C+2Cl₂=SiCl₄+CO₂; ΔH° (1223 K)=−141.7 kJ/mol

Glow Discharge Mass Spectrometry (GDMS) analysis indicates that siliconalso exists in boron carbide (0.38% (wt) in one batch of boron carbidesampled). Hence, the SiCl₄ in the BCl₃ stream may also be attributed tothe silicon impurity in B₄C (the source material of BCl₃).

Therefore, there is a need to have a reactor for synthesizing BCl₃ inthe absence of a silicon source and/or a process for the removal of anySiCl₄ impurities that may form during the synthesis processes.

BRIEF SUMMARY OF THE INVENTION

Accordingly, a process for the production of boron trichloride (BCl₃) bythe reaction of boron carbide (B₄C) with chlorine at a temperature of800° C. to 1200° C. is disclosed herein using a reactor that eithereliminates the silicon source resulting from the reactor by forming thereactor from a non-quartz material or applying a protective barrier tothe quartz surface which is inert to chlorine attack at reactionconditions to prevent/minimize silicon tetrachloride (SiCl₄) formation.

Alternatively, in the case that quartz has to be used, employing anappropriate reactive material or adsorbent material to remove SiCl₄ andother silicon chlorides from the BCl₃ stream;

Alternatively, if the silicon contribution from the quartz reactor isminimized, and the formation of SiCl₄ is via the silicon impurity inB₄C, similar to the above method, an appropriate reactive material oradsorbent material can be used to remove SiCl₄ from the BCl₃ productstream. In this instance, the present invention teaches the use of ahydride reducing agent to convert SiCl₄ to SiH₄, which is easier toseparate from BCl₃ than SiCl₄. The hydride can readily be treated fordisposal in the gas phase through controlled oxidation (e.g. exposure oflow concentrations to air or burning in the presence of a fuel source),scrubbing with a liquid phase oxidizing medium (e.g. aqueous KMNO₄ orNaOCl), scrubbing with a solid phase medium (e.g. Cu(OH)₂), or otheracceptable method.

In yet another embodiment, SiCl₄ may be further removed from a BCl₃stream by freeze purification. Atmospheric pressure boiling points forBCl₃ and SiCl₄ are about 12.6° C. and 57.65° C., respectively. Freezingpoints are about −107.3° C. and −68.74° C. This suggests that solidSiCl₄ may be removed by condensing and cooling the BCl₃ product.

Additional embodiments and features are set forth in the descriptionthat follows, and in part will become apparent to those skilled in theart upon examination of the specification or may be learned by thepractice of the disclosed embodiments. The features and advantages ofthe disclosed embodiments may be realized and attained by means of theinstrumentalities, combinations, and methods described in thespecification.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the preferred embodiments of the presentinvention, and together with the description serve to explain theprinciples of the invention.

In the Drawings

FIG. 1 is a perspective view of the column reactor device of the presentinvention, with portions cut away to reveal the internal structure asassembled.

FIG. 2A is an enlarged, detailed view of the protective coating of thecolumn reactor device of FIG. 1 indicated by dashed lines in FIG. 1.

FIG. 2B is an enlarged, detailed view of the present invention similarto the view shown in FIG. 2A, but having multiple protective coatingsoverlying one another.

FIG. 2C is an enlarged, detailed view of the protective coating of thepresent invention similar to the view shown in FIG. 2A having analternative protective barrier in contact with the surface of theprotective coating.

FIG. 2D is an enlarged, detailed view of the present invention as shownin FIG. 2C illustrating an alternative embodiment wherein the protectivecoating is absent and the protective barrier is in direct contact withthe interior column reactor sidewalls and the reactive media.

FIG. 3 is a side, cross-sectional view of the filter of FIG. 2D, withthe addition of a filter bed.

FIG. 4 is a top plan view of the column reactor device shown in theposition illustrated in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

It has now been discovered that the presence of the silicontetrachloride (SiCl₄) impurity found in boron trichloride (BCl₃) can beminimized during the synthesis of BCl₃ and/or removed from the BCl₃product stream thus producing a purified BCl₃.

In accordance with the present invention, the silicon sourceattributable to the reactor is either eliminated by constructing areactor column from a non-quartz material which is inert to chlorineattack at reaction conditions or minimized by inserting an protectivecoating or barrier between the interior sidewall of the column reactorand the reactive material used for the synthesis of BCl₃, such as butnot limited to boron carbide. This protective coating or barrier therebyminimizes the formation of impurities (such as SiCl₄) that are generatedby the reaction of the interior quartz reactor walls with reactivechemical species within the reactor. It should be understood andappreciated that the embodiments and/or features of the presentinvention disclosed herein may be freely combined with one another.

In one embodiment, the reactor 10 of the present invention, as shown inFIG. 1, makes use of an inert non-reactive material 14 that showsresistance to chlorine attack, such as but not limited to graphite,graphene, or silicon oxynitride, or refractory ceramic materials to forma dense nonporous thin protective coating or layer on the inner surface12 of quartz column 16. The ceramics useful in this invention includebut are not limited to silicon carbide, zirconium carbide, zirconiumnitride, silicon nitride, or boron nitride. Such a protective coating 14must have close thermal properties to quartz to avoid/minimizelamination and/or stress/tension due to thermal expansion at elevatedtemperatures.

The protective coating 14, best seen in FIG. 2A, is juxtaposed betweenthe interior surface 12 of the interior reactor 10 wall and the reactivematerial R, such as, but not limited to a boron compound such as boroncarbide (B₄C). Typically, protective coating 14, such as a graphenecoating is formed by a chemical vapor deposition (CVD) process usingmethane or ethanol as a precursor at temperatures near 1000° C. on theinterior surface 12 of quartz column 16 having an interior surface andan exterior surface. Alternatively, the protective coating 14 may be asilicon oxynitride layer formed on the interior surface 12 of the quartzmaterial by rapidly flowing ammonia gas at 1200° C. A dense refractoryceramic coating typically is formed by a CVD process with appropriateprecursors. For instance, a thin layer of boron nitride can be depositedon a quartz column surface by the chemical reaction between borontrichloride (or other boron compounds and ammonia. Alternatively, one ormore additional coating layers (not shown) may be formed over theprotective coating 14 on the interior surface 12 of the quartz column16.

If reactor 10 includes one or more different coatings 17 overlyingand/or underneath the surface 15 of protective coating 14′ deposited oninterior surface 12 of the quartz column 16, as shown in FIG. 2B, thevarious coatings may be formed as adjacent layers overlying one anothersequentially, or one or more of the coatings may penetrate into or eventhrough one or more of the other coatings. Accordingly, the variouscoatings may be fairly described as being formed generally “on” or“over” the column, regardless of how or to what extent any given coatingcontacts any of the other coatings and/or the column itself. Similarly,when a material is described as being applied generally to the column,the material may be applied directly to the quartz column, or thematerial may be applied to the quartz column over one or more coatingsalready present on the quartz column.

In another embodiment, shown in FIG. 2C, the surface temperature ofquartz column 16 can be reduced by packing a concentric ring of largerdiameter, and/or less porous, and/or lower reactive particles, such as,but not limited to boron nitride, pure boron, etc. into quartz column 16to form a barrier 18 between the protective coating 14 of quartz column16 and reactive material R, such as boron carbide (B₄C). Alternatively,barrier 18′ can be formed in direct contact with the interior surface 12of column 16, as shown in FIG. 2D. This concentric ring of material, asbest shown in FIG. 4, may be formed in juxtaposition with the interiorsidewall surface 12 of quartz column 16 or it may have a graphite,graphene, or silicon oxynitride, or refractory ceramic materialsinterposed between it and the quartz surface, as shown in FIG. 2C. Inanother embodiment, not shown, a barrier may be formed within column 16using a non-reactive tube, as opposed to loose particles which arepacked into column 16. In this embodiment the exterior diameter of thetubular barrier would be slightly less than the inner diameter of column16 so that when slid into place, the exterior surface of the tubularbarrier would be in contact with the interior surface of column 16. Inthe event the exterior diameter of the tubular barrier is significantlyless than the interior diameter of column 16 then the concentric annularspace or gap that is formed can further be pack with a ring of largerdiameter, and/or less porous, and/or lower reactive particles such asthose that were used to describe barrier 18 above.

By using an embodiment of reactor 10 as disclosed herein, BCl₃ can beprepared by introducing chlorine gas C through gas inlet 20 thus passingover boron carbide and optionally carbon, packed within quartz column16, which is heated, using inductive heating H (FIG. 1), to elevatedtemperatures of at least 800° C. to 1,200° C. Once the reaction isestablished, the reaction zone propagates slowly down the columngenerating BCl₃ at the reaction zone. The chlorination reaction resultsin the formation of BCl₃ and the presence of the silicon tetrachloride(SiCl₄) impurity typically found in BCl₃ is minimized during thesynthesis of BCl₃ as a result of the protective barrier that isestablished between the reactive materials and the quartz substrate. Aswill be disclosed in further detail below due to the presence of asilicon source in the reactive material R, any SiCl₄ that is eventuallyformed may be removed using the process described herein therebyproducing a purified BCl₃ which exits reactor 10 by way of gas outlet22.

As discussed previously, regardless of the steps taken to eliminate thesilicon source that results from the reactor, SiCl₄ impurities may stillform due to the presence of a silicon source in the B₄C. Therefore, thepresent invention further contemplates processes for purifying the BCl₃product to remove any SiCl₄ impurity formed regardless of whether theinterior sidewalls of the column are protected by a coating or a barrieras discussed above. The following embodiment as shown in FIG. 3contemplates having a protective coating or barrier; however, oneskilled in the art will also recognize that the purification bed, asdisclosed herein, could also be used in a standard quartz column (notshown). As shown in FIG. 3 and in accordance with the present inventiona thin pure boron zone 120 or purification bed is formed at the bottomof reactor 110 and maintained within quartz column 116 using a ceramicfrit or alternatively boron is placed in a heated separate bed (notshown) to react with any SiCl₄ to form BCl₃ and solid Si, so that SiCl₄impurity is removed from the BCl₃ stream. Alternatively, propermolecular sieve materials having good affinities for SiCl₄ andappropriate pore size to let SiCl₄ molecules diffuse into the pores andbe adsorbed on the internal surfaces of the adsorbent materials butexclude BCl₃ molecule to enter the pores (the kinetic diameter of SiCl₄and BCl₃ is 5.81 Å and 6.00 Å, respectively) can be utilized.

Purification bed 120 may also be formed using a reactive elementalmaterial, mixed material or other compound to react with SiCl₄ to form,e.g. M_(x)Cl_(y), wherein x=1-4 and y=1-8 and elemental silicon suchthat the SiCl₄ present as an impurity in BCl₃ is substantially removed.The reactive material is at least partially consumed and acts as a SiCl₄getter. The byproduct or byproducts of reaction may need to be separatedfrom BCl₃, but this should be more convenient than separation of SiCl₄.Preferred materials are elemental titanium (e.g. Ti sponge), 90%NaCl/10% elemental boron, elemental zinc (e.g. molten), and alumina(Al₂O₃).

A hydride reducing agent may further be used to convert SiCl₄ to SiH₄,which is easier to separate from BCl₃ than SiCl₄. The hydride canreadily be treated for disposal in the gas phase through controlledoxidation (e.g. exposure of low concentrations to air or burning in thepresence of a fuel source), scrubbing with a liquid phase oxidizingmedium (e.g. aqueous KMNO₄ or NaOCl), scrubbing with a solid phasemedium (e.g. Cu(OH)₂), or other acceptable method. Without wishing to bebound by theory, general reaction schemes may include, for example:

-   -   SiCl₄+4MH→SiH₄+4MCl    -   SiCl₄+2MH₂→SiH₄+2MCl₂    -   SiCl₄+MM′H₄→SiH₄+MCl+M′Cl₃    -   SiCl₄+4MR₂H→SiH₄+4MR₂Cl    -   SiCl₄+4MM′R₃H→SiH₄+4MCl+4M′R₃        Where M comprises an alkaline earth metal, alkali metal or other        main group metal or metalloid, and R comprises a hydrocarbyl        group.

The hydride reducing agent may include, but is not limited to, one ormore of the following: LiH, NaH, KH, CaH₂, LiAlH₄, NaBH₄,diisobutylaluminum hydride (DIBAL), and lithium triethylborohydride(LiB(Et)₃H). Other hydride reducing agents not included in this list mayalso be effective. Ideally, the reducing agent will have a highselectivity for SiCl₄ over BCl₃ and will yield a byproduct or byproductsthat do not have a significant negative impact on subsequent processing.

The best choice from the standpoint of reactivity and byproductformation may be NaBH₄, as this is less reactive than the alkaline andalkali earth hydrides and would generate NaCl and BCl₃. DIBAL may alsobe a good choice, as the byproduct, diisobutylaluminum chloride, is avery high boiling liquid that would not generate solids in the process.

SiCl₄ may be further removed from a BCl₃ stream by freeze purification.Atmospheric pressure boiling points for BCl₃ and SiCl₄ are about 12.6°C. and 57.65° C., respectively. Freezing points are about −107.3° C. and−68.74° C. This suggests that solid SiCl₄ may be removed by condensingand cooling the BCl₃ product.

Without further elaboration it is believed that one skilled in the artcan, using the description set forth above, utilize the invention to itsfullest extent.

Having disclosed several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of thedisclosed embodiments. Additionally, a number of well known processesand elements have not been described in order to avoid unnecessarilyobscuring the present invention. Accordingly, the above descriptionshould not be taken as limiting the scope of the invention.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimits of that range is also specifically disclosed. Each smaller rangebetween any stated value or intervening value in a stated range and anyother stated or intervening value in that stated range is encompassed.The upper and lower limits of these smaller ranges may independently beincluded or excluded in the range, and each range where either, neitheror both limits are included in the smaller ranges is also encompassedwithin the invention, subject to any specifically excluded limit in thestated range. Where the stated range includes one or both of the limits,ranges excluding either or both of those included limits are alsoincluded.

As used herein and in the appended claims, the singular forms “a”, “an”,and “the” include plural referents unless the context clearly dictatesotherwise. Thus, for example, reference to “a process” includes aplurality of such processes and reference to “the dielectric material”includes reference to one or more dielectric materials and equivalentsthereof known to those skilled in the art, and so forth.

Also, the words “comprise,” “comprising,” “include,” “including,” and“includes” when used in this specification and in the following claimsare intended to specify the presence of stated features, integers,components, or steps, but they do not preclude the presence or additionof one or more other features, integers, components, steps, acts, orgroups.

What is claimed is:
 1. A quartz column reactor device, for use informing a purified boron trichloride comprising: a gas inlet and a gasoutlet; a quartz column reactor having inner sidewalls and outersidewalls; and a non-reactive barrier formed over the surface of saidinner sidewalls.
 2. The reactor device of claim 1, wherein saidnon-reactive barrier is selected from the group consisting of graphene,or silicon oxynitride, or refractory ceramic materials.
 3. The reactordevice of claim 2, wherein said refractory ceramic material is selectedfrom the group consisting of silicon carbide, zirconium carbide, orzirconium nitride, or silicon nitride, or boron nitride.
 4. The reactordevice of claim 2, wherein said reactive barrier is at least one layerthick.
 5. The reactor device of claim 1, further comprising apurification bed wherein said purification bed is formed using molecularsieve materials having affinities for SiCl₄ and appropriate pore sizesto exclude BCl₃ molecules while allowing SiCl₄ molecules to diffuse intosaid pores and be adsorbed on the internal surface of said adsorbentmaterial.
 6. The reactor device of claim 5, wherein said pore size ofadsorbent material is less than 6.00 Å.
 7. The reactor device of claim5, wherein said bed comprises a reactive material, mixed material orother compound to react with SiCl₄.
 8. The reactor device of claim 7,wherein said reactive materials comprise elemental titanium, 90%NaCl/10% elemental boron, elemental zinc, alumina (Al₂O₃), a hydridereducing agent and/or combination thereof.
 9. The reactor device ofclaim 8, wherein said reducing agent comprises LiH, NaH, KH, CaH₂,LiAlH₄, NaBH₄, diisobutylaluminum hydride (DIBAL), lithiumtriethylborohydride (LiB(Et)₃H) and/or combinations thereof.
 10. Thereactor device of claim 1, wherein said non-reactive barrier formed overthe surface of said inner sidewalls is formed by positioning anon-reactive tube having exterior sidewalls and interior sidewallsconcentrically within the column.
 11. The reactor device of claim 10,wherein said exterior sidewalls of said tube are contact with saidinterior sidewalls of said column.
 12. The reactor device of claim 10,wherein the exterior diameter of said non-reactive tube is less than theinterior diameter of said column so that when said non-reactive tube ispositioned with said column an concentric annular space is formed and amaterial having a large diameter and/or less porous and/or less reactiveparticles are packed into said annular space.
 13. A quartz columnreactor device, for use in forming a purified boron trichloridecomprising: a gas inlet and a gas outlet; a quartz column reactor havinginner sidewalls and outer sidewalls; and a purification bed wherein saidpurification bed is formed using a reactive material and/or an adsorbentwherein said adsorbent comprises molecular sieve materials havingaffinities for SiCl₄ and appropriate pore sizes to exclude BCl₃molecules while allowing SiCl₄ molecules to diffuse into said pores andbe adsorbed on the internal surface of said adsorbent material.
 14. Thereactor device of claim 13, wherein said purification bed is formedusing pure boron.
 15. The reactor device of claim 13, wherein said poresize of adsorbent material is less than 6.00 Å.
 16. The reactor deviceof claim 13, wherein said purification bed comprises a reactivematerial, mixed material or other compound to react with SiCl₄.
 17. Thereactor device of claim 13, wherein said reactive materials compriseelemental titanium, 90% NaCl/10% elemental boron, elemental zinc,alumina (Al₂O₃), a hydride reducing agent and/or combination thereof.18. The reactor device of claim 17, wherein said reducing agentcomprises LiH, NaH, KH, CaH₂, LiAlH₄, NaBH₄, diisobutylaluminum hydride(DIBAL), lithium triethylborohydride (LiB(Et)₃H) and/or combinationsthereof.
 19. The reactor device of claim 13, wherein said quartz columnfurther comprises a refractory ceramic material selected from the groupconsisting of silicon carbide, zirconium carbide, or zirconium nitride,or silicon nitride, or boron nitride formed over the surface of saidinner sidewalls.